3D Rotational X-ray (3DRX) imaging can be used to intraoperatively
acquire 3D volumes depicting bone structures in the patient. Registration of 3DRX to MR images, containing soft tissue
information, facilitates image guided surgery on both soft tissue and
bone tissue information simultaneously. In this paper, automated noninvasive registration using maximization of mutual information is compared to conventional interactive and invasive point-based registration using the least squares fit of corresponding point sets. Both methods were evaluated on 3DRX images (with a resolution of 0.62x0.62x0.62 mm<sup>3</sup>) and MRI images (with resolutions of 2x2x2 mm<sup>3</sup>, 1.5x1.5x1.5 mm<sup>3</sup> and 1x1x1 mm<sup>3</sup>) of seven defrosted spinal segments implanted with six or seven markers. The markers were used for the evaluation of the registration transformations found by both point- and maximization of mutual information based registration. The root-mean-squared-error on markers that were left out during registration was calculated after transforming the marker set with the computed registration transformation. The results show that the noninvasive registration method performs significantly better (p≤0.01) for all MRI resolutions than point-based registration using four or five markers, which is the number of markers conventionally used in image guided surgery systems.